Composition-tunable transition metal dichalcogenide nanosheets via a scalable, solution-processable method

The alloying of two-dimensional (2D) transition metal dichalcogenides (TMDs) is an established route to produce robust semiconductors with continuously tunable optoelectronic properties. However, typically reported methods for fabricating alloyed 2D TMD nanosheets are not suitable for the inexpensive, scalable production of large-area (m2) devices. Herein we describe a general method to afford large quantities of compositionally-tunable 2D TMD nanosheets using commercially available powders and liquid-phase exfoliation. Beginning with Mo(1−x)WxS2 nanosheets, we demonstrate tunable optoelectronic properties as a function of composition. We extend this method to produce Mo0.5W0.5Se2 MoSSe, WSSe, and quaternary Mo0.5W0.5SSe nanosheets. High-resolution scanning transmission electron microscopy (STEM) imaging confirms the atomic arrangement of the nanosheets, while an array of spectroscopic techniques is used to characterize the chemical and optoelectronic properties. This transversal method represents an important step towards upscaling tailored TMD nanosheets with a broad range of tunable optoelectronic properties for large-area devices.

A voltage of 10 V was applied for 24 h (WE as cathode), during which the solution begins to turn yellow at the anode, and the pellet begins to slowly expand and slough off.After the pellet has become a fluffy powder at the bottom of the beaker, the powder and remaining pellet were carefully collected and washed thoroughly with ethanol via vacuum filtration with a nylon filter (pore size 0.45 µm).The solid material was transferred to a 50 mL centrifuge tube with 10 mL of NMP (Acros Organics; 99+% for spectroscopy) before bath sonication (Ultrasonic bath USC T, VWR, 45 kHz) in water at 25°C for 1-2 hours.
Finally, to remove any unexfoliated bulk (including remains of the pellet, which can be reused), the solution was centrifuged for 30 min at 120 rcf using an Eppendorf centrifuge 5810 equipped with a FA-45-6-30 rotor.The top 8 mL of supernatant were collected and transferred to a new container.These processing conditions typically gave nanoflakes of mostly 1-5 layers with a lateral dimension of 500 nm to 1 µm, as has been previously established. 1Note that additional centrifugation or filtration steps could be performed to isolate a narrower dispersion of nanosheets as needed.
Thin Film Formation: Thin films were made via a liquid/liquid interface created between deionized water and hexane (Sigma-Aldrich; >99%) using a previously-described approach, 2 slightly modified as described here (eater, hexane system, NMP for dispersion solvent). 1Transfer to substrate was accomplished by aspirating the organic phase and then either aspirating the water phase to deposit the film onto a pre-positioned substrate (for FTO glass) or via a stamping method (for FET substrates) wherein the substrate is manipulated with a suction pen and pressed into the film, transferring the material onto the substrate.Films were then annealed at 200 °C for 120 min in a vacuum oven to remove excess solvent.
UV-Visible Spectroscopy: UV-Vis spectra were acquired using a Shimadzu UV 3600 spectrometer from 825-300 nm using an integrating sphere with step size of 1 nm and slit width of 5 nm.Measurements of solutions were taken using a quartz cuvette directly in transmission mode.Dispersions of nanosheets in NMP were diluted in NMP.NMP was used for a blank.Absorbance was calculated as shown in equation ( 1): %Absorbance = 2 -log 10 (%Transmission) (1) Raman Spectroscopy: Raman spectra and PL spectra were obtained using a Horiba Xplora Plus Raman microscope with 532 nm radiation (40 mW).Raman spectra were acquired from 100-1800 cm -1 using a 100x objective, slit of 200 µm, hole of 500 µm, a grating with 2400 gr/mm, 10% filter, 10s of acquisition, and 5 accumulations.Note that the diameter of our excitation beam is large compared to average nanosheet size and the nanosheets are relatively densely packed on the substrate.This means the signal observed will be an average over many nanosheet sizes.
Photoluminescence Spectroscopy: PL spectra were obtained from 550-950 nm using a 100x objective, a grating with 600 gr/mm, slit of 200 µm, hole of 500 µm, 25% filter, 4s of acquisition, and 4 accumulations.PL spectra were normalized according to Raman signals appearing around 580 nm to adjust for material content and then to [0,1] for display clarity.
X-ray Photoelectron Spectroscopy: XPS spectra were acquired using a PHI Versa Probe II (Physical Instruments AG, Germany).
Analysis was performed using a monochromatic Al-Kα X-ray source operated at 50 W.The spherical mirror analyzer was set at 45° take-off angle with respect to the sample surface.The pass energy was 46.95 eV yielding a full width at half maximum of 0.91 eV for the Ag 3d 5/2 peak.
X-Ray Diffraction: Powder XRD measurements were taken in Debye-Scherrer geometry (scanning mode) using Cu-Kα radiation on a Bruker D8 Discover Plus instrument equipped with a rotating anode and a Dectris Eiger2 500K detector.Samples were loaded into 0.5 mm borosilicate capillaries and spun during data acquisition.Lattice parameters were measured by fitting peak intensities.
FET Measurements: FET transistors were made by using LLISA to deposit 1-2 layers of TMD nanosheets onto commercially available substrates (Fraunhofer ISE): Au coated SiO 2 (210 nm) transistor substrates were used with 10 mm channel widths (W), an insulator capacitance (C i ) of 1.8 × 10-8 F., and channel lengths of 5 μm.FET measurements were carried out in a nitrogen atmosphere using a custom-built probe station and a Keithley 2612A dual-channel source measure unit.Drain voltage was scanned from 0 to 40 V with gate voltages from 0 to 40 V.
STEM EDX analyses: STEM EDX mappings and HAADF images were acquired on a Thermo Fisher Scientific Talos F200S microscope operated at 200 kV.STEM HAADF images were obtained using a probe current of 100 pA, a camera length of 77 mm and a dwell time of 1-2 µs.The elemental net count maps were processed with Velox software and the quantification used the Cliff-Lorimer method.Atomic-resolution STEM images were acquired on a Thermo Fisher Scientific Titan Themis microscope operated at 80 kV and equipped with a high-brightness field emission gun (X-FEG) and a 4-segment STEM detector used for iDPC imaging.The aberrations of the probe were corrected with a CEOS DCOR system up to the 4 th order and a convergence angle of 20 mrad were used.HAADF and iDPC imaging were acquired simultaneously using a probe current of 30 pA, a camera length of 115 mm and a dwell time of 2 µs.X-ray diffraction (XRD) of the pure and mixed pellets shows well defined peaks consistent with successful formation of crystalline domains.The small difference in MoS 2 and WS 2 lattice parameters (<0.5%) makes distinguishing the peaks challenging is Fig.S1a.Accordingly, a closer look at two (002) peaks at 14.35-14.40shows a systematic shift to higher 2θ from WS 2 to Mo 0.5 W 0.5 S 2 to MoS 2 (Fig. S1b).This is consistent with an increase of lattice parameters with the incorporation of W atoms.By fitting the intensities shown in Fig. S1a, it is possible to calculate the unit cell parameters, as shown for the c-planes in Fig. S1c.
Theoretical works suggest that the lattice parameters should follow Vegard's law for alloyed materials, which empirically states that combining two materials with the same crystal structure should yield a linear combination of their lattice parameters according to the composition. 3Indeed, the change in lattice parameter shows a perfectly linear relationship as a function of W atomic percent, appearing to follow Vegard's law as theoretically predicted.which were mixed with 1:1 atomic ratio but were not mechanically ground prior to pellet formation.
While some nanosheets show even distribution of Mo and W, indicating ternary alloy formation, others appear to be pure binary phases.This emphasizes the importance of thorough grinding of the powders before pressing them into pellets.To concretely confirm alloy formation, Raman spectra for the four ternary alloys is shown in Fig. S6a.][7][8][9][10][11][12][13] Panel one and two (green and purple) show evidence for the typical group VI metal-alloyed TMDs, Mo 0.5 W 0.5 S 2 and Mo 0.5 W 0.5 Se 2 .Notably this means that both n-type and p-type alloys can be fabricated and tuned using this method.Additionally, panel three and four (cyan and blue) confirm that chalcogen-alloyed TMDs are also attainable, finishing off the possible combinations.
The optoelectronic properties are studied using UV-Vis (solid lines) and PL (broken lines) in Fig. S6b.As with the Raman spectroscopy the UV-Vis spectra show unique absorption signals, displaying altered exciton peaks and regions of increased absorption compared to the pure TMDs.Accordingly, PL is also shifted compared to the pure materials and can be recorded for all combinations, with the exception of the Mo 0.5 W 0.5 Se 2 alloy.Given the red-shifted nature (compared to MoSe 2 and WSe 2 ) of the excitonic peak and the expected Stokes shift of the PL signal, it is possible that PL is present but not detectable in the set-up used in this work.Another explanation could be the general instability of selenide-based materials. 14Indeed, if too many defects are present charge recombination may occur too quickly to be detected in this very simple apparatus.XRD analysis of the annealed Mo 0.5 W 0.5 SSe pellet is shown along with XRD for the pure materials that were mixed to form the pellet.From the wide view it can be seen that all three materials give sharp peaks, consistent with the formation of crystalline domains.The inset shows a close up of the (1 0 3), (0 0 6), and (1 0 5) peaks, respectively left to right.For each reflection the quaternary alloy lies directly in the middle of the two pure materials, seemingly following Vegard's law for alloyed materials. 2As the lattice parameters for MoS 2 and WSe 2 are quite different (>5%), a distinction is more clearly seen compared to the case of the Mo 0.5 W 0.5 S 2 pellet (described in Fig. S1).The Raman spectrum displayed in Fig. S8b is complex as is expected for a material with many vibrational modes including: Mo-S-W, W-S, Mo-S, Mo-W-Se, W-S-Se, Mo-S-Se. 6Importantly this spectrum is not only distinct from either pure material, but also from all of the ternary alloys previously discussed (see Fig. S6).

Fig. S1 .
Fig. S1.Powder XRD (source: Cu-K-alpha 1) patterns of pellets made of pure MoS2 (purple), pure WS2 (green), and 1:1 MoS2:WS2 (blue).(a) Wide view shows sharp, high-intensity peaks confirming the crystallinity of the three pellets.(b) Close up of the (002) reflection shows systematic peak shifting consistent with changed lattice parameters.(c) Lattice parameter for the c-planes as a function of W atomic percent.A linear fit (red, solid line) confirms Vegard's law for alloys.

Fig. S2 .
Fig. S2.Examples of alloyed TMDs coated on a variety of substrates using LLISA deposition (a-d) and drop casting (e).(a) Flexible substrate, PET, suitable for roll-to-roll applications. 4(b) Rigid Au-bottom patterned Si/SiO 2 transistor with close up of 2.5µm gate (yellow box) shown in (c).(d) Rigid FTO-coated glass for typical photoelectrochemical testing. 1 (e) Glass beads to demonstrate atypical substrate coating.

Fig. S4 .
Fig. S4.STEM EDX of exfoliated nanosheets made from alloyed powders which were not mechanically ground.STEM (a) HAADF imaging and (b) EDX composite elemental mapping (net counts) for Mo x W x S 2 nanoflakes made from MoS 2 and WS 2 powders

Fig. S9 .
Fig. S9.Characterization for thin film of exfoliated Mo 0.5 W 0.5 SSe nanosheets.(a) XPS spectra for a thin film of exfoliated Mo 0.5 W 0.5 SSe for core level Mo 3d (top left, blue), W 4f (bottom left, green), S 2p (top right, red), and Se 3d (bottom right, yellow), confirming the presence of the four expected elements.(b) Raman spectrum for a thin film of exfoliated Mo 0.5 W 0.5 SSe.